1. Field of the Invention
The present invention relates to an optical pickup apparatus, and more particularly to an optical pickup apparatus which is adapted for a plurality of sorts of optical discs, for example, a CD (compact disc), a DVD (digital versatile disc) and a BD (high-density optical disc employing blue laser beams such as blu-ray disc).
2. Description of Related Art
In, for example, an optical pickup apparatus of plural-wavelength and single-lens type which is adapted for a plurality of sorts of optical discs (for example, a CD, a DVD and a BD) of different wavelengths for use by employing a single objective lens, when it is intended to obtain satisfactory reproduced signals from all the optical discs, a beam spot which is formed by the objective lens needs to be brought into a circular shape of small diameter. Besides, the inclinations and positional deviations of laser beams relative to an optical axis need to be corrected so that all the laser beams may enter the objective lens along the same optical paths.
Regarding the spot shape, Patent Document 1 (being JP-A-2005-309351), etc. have proposed an optical pickup apparatus wherein a rim intensity (that is, the peripheral intensity ratio of a light beam entering the objective lens) is corrected by employing a beam shaping element (for example, a prism or a cylindrical lens) which converts the laser beam from an elliptic beam into the circular beam. Regarding the optical paths of the laser beams, Patent Documents 2, 3 and 4 (being JP-A-2000-99978, JP-A-2002-208163 and JP-A-2003-30887, respectively) have proposed optical pickup apparatuses of dual-wavelength and single-lens type wherein the optical paths to the objective lens are synthesized by a prism which has a beam shaping function. The prism includes a surface which transmits the laser beam of one wavelength therethrough and which reflects the laser beam of the other wavelength therefrom, and it is configured so as to synthesize the optical paths by the surface. Besides, Patent Document 5 (being JP-A-2005-122878) has proposed an optical pickup apparatus wherein an upstand mirror which is made of a liquid crystal element for making wavefront corrections by a predetermined refractive index distribution is employed as means for correcting the laser beams that are to enter the objective lens. Patent Document 6 (being JP-A-2006-155827) has proposed an optical pickup apparatus wherein a variable focus type liquid crystal element whose refractive index is electrically controllable is disposed in the vicinity of an upstand mirror.
However, when the beam shaping elements as proposed in Patent Document 1 are arranged for the respective wavelengths, the whole optical system of the optical pickup apparatus becomes large in size and complicated in structure. Besides, in case of each of the optical pickup apparatuses of the dual-wavelength and single-lens type as proposed in Patent Documents 2 to 4, the laser beam of the wavelength which is reflected from the surface of the prism for synthesizing the optical paths is not subjected to beam shaping. That is, only the beam shaping of one wavelength can be performed, and a high output is required for the laser light source of the other wavelength. Each of the optical pickup apparatuses proposed in Patent Documents 5 and 6 is configured so as to change the refractive index distribution or refracting power of the liquid crystal element, so that the optical paths to the objective lens cannot be synthesized by the liquid crystal element.